ISOLATION AND PURE-CULTURE METHODS 23 



sterility from bacteria and fungi is desired it would be necessary to steam the 

 soil for a much longer time. The steamed soil is also almost wholly free from 

 live seeds of weeds while the untreated soil was considerably infested with vari- 

 ous common weeds. 



A sterilizer may be arranged more or less after this pattern, but 

 with particular reference to the conditions at hand. A sterilizer of 

 this type may also be used for pots and saucers already filled with 

 soil. A better pressure of steam may be secured, of course, if the 

 sterilizer is directly connected with the boiler. For summer work, 

 moreover, it is not desirable to have the sterilizer connected with 

 the general heating system. 



Soil sterilized by dry heat requires a very high temperature, and 

 is unquestionably somewhat injured in the process. On the other 

 hand, it must be remembered that soil which has been steam-ster- 

 ilized encourages upon reinfection the growth of such saprophytic 

 organisms as Mucor and Penicillium, and possibly such hemisapro- 

 phytes as Rhizoctonia and other fungi causing root diseases. Care 

 must be taken, therefore, not to permit these organisms to get a 

 start in the soil before normal bacterial action has begun. 



IV. PREPARATION OF CULTURE MEDIA 

 LIQUID MEDIA 



Except in investigations where a medium of known composition 

 is required, or in drop cultures and the like, liquid media are less 

 used for cultural purposes with the fungi than with the bacteria. 

 In many physiological studies, however, such media are desirable 

 or indispensable, and as a rule these liquid media form the nutrient 

 bases for the making of most of the gelatinous solid media employed 

 in mycological work. 



Plant decoctions are undoubtedly of the first value for work 

 with the fungi, and with these organisms they may entirely replace 

 bouillon, considered so essential in the culture of bacteria. Some 

 of the most nutritious and convenient plant decoctions may be made 

 from the sugar beet, white potato, carrot, pods and stems of bean, 

 or vetch, prunes, apples, celery, and various other plants or plant 

 products. In order to secure more or less uniformity in the com- 

 position of these decoctions, for every 1000 cc, of water used it 



